Many view the ACA—which included mental health services and treatment for substance abuse as required benefits for health plans—as the most significant of three legislative pillars of that progress.

“This is another reason why the immediate repeal of the Affordable Care Act creates a cascade of unintended consequences that are not going to be acceptable to the vast majority of people,” said Paul Gionfriddo, president and CEO of Mental Health America.

Nearly nine years ago Congress adopted legislation intended to impose parity in health coverage for mental health services. The Obama administration implemented the law in 2013 with final regulations that required health plans to have the same deductibles, cost-sharing and coverage limits for mental health services as offered for medical and surgical services.

Congress added more protections late last year in the 21st Century Cures Act, which was signed by President Barack Obama in December. The law includes tougher enforcement of parity requirements and calls for HHS to help health plans comply.

Both the 2008 Mental Health Parity Equity Act and the 21st Century Cures Act won bipartisan support. But the effectiveness of both laws would be severely hindered by ACA repeal.

“The irony is you can have a decent parity law, which we do, the oversight of the 21st Century Cures Act, but if you take out the Affordable Care Act's essential benefits provision, then you could effectively gut it without even intending to,” Gionfriddo said. “Somebody who casts a repeal vote without coming up with an alternative that's going to be inclusive of people with mental health issues would be doing a lot to undermine this new legislation.”

Among the supporters of repealing the ACA is Rep. Tim Murphy (R-Pa.), who penned many of the mental health provisions folded into the 21st Century Cures Act. Murphy contends the ACA has failed to improve access to mental healthcare even as the rate of Americans without health insurance has fallen to the lowest on record.

“The Affordable Care Act did not improve the federal mental health system, it simply expanded ineffective, inefficient policies that have not delivered treatment before tragedy,” said Murphy spokeswoman Carly Atchison. "Rep. Murphy welcomes the opportunity to work across the aisle, and remains committed to ensuring that those with serious mental illness can get the treatment they need with insurance that isn't passive, but instead works in partnership with the patient.”

So far the Trump administration has yet to unveil any specific plans for what would replace the ACA. On Tuesday, a group of Republican senators introduced a bill dubbed the Patient Freedom Act. It would retain some of ACA's provisions, including requiring insurers to cover mental health and substance use disorder services and maintaining parity rules for plans sold on the individual market.

The proposed bill would also allow states to keep in place the ACA's Medicaid expansion or create a “state and market-based alternative."

The Trump administration and congressional leaders, meanwhile, favor turning Medicaid into a block grant program, which would give states a fixed amount of money to provide healthcare for low-income residents.

Previous Medicaid block grant proposals have assumed significant savings from the model. Policy experts say that means states would have to reduce eligibility, narrow the scope of benefits or impose cost-sharing requirements.

How Medicaid plays out is crucial, said Dr. Maria Oquendo, president of the American Psychiatric Association, because people who suffer from mental illness are disproportionately likely to be unemployed or have low-wage jobs with no health benefits.

Medicaid is the single largest payer for behavioral health services in the U.S. In 2009 the program accounted for 26% of all spending on mental health services, according to a June 2015 report by the Medicaid and CHIP Payment and Access Commission.

Coverage of behavioral health services provided under Medicaid were found to be generally more comprehensive than services covered by plans sold in the ACA marketplaces, according to a June 2015 study by the Kaiser Family Foundation.

“The whole array of intensive community-based mental health services that people with significant psychiatric disabilities need—that's all through public mental health and that's all Medicaid-financed,” said Jennifer Mathis, deputy legal director at the Bazelon Center for Mental Health Law. “I don't think it's consistent to profess a great desire to improve mental health and to help people with mental illnesses and at the same time to talk about block granting the entitlements that they have and essentially eliminating the entitlement they have now through Medicaid.”

Repealing the ACA's premium subsidies and the Medicaid expansion without creating other avenues for coverage would mean 30 million more Americans would be uninsured in 2019, according to the Urban Institute. That number assumes 13 million fewer people would be covered by Medicaid and CHIP.

“The risks that we're facing now with ACA repeal are people losing coverage altogether,” Mathis said. “And parity doesn't get you anything if you don't have coverage to begin with.”